This invention relates to a contact arrangement for vacuum switches having an axial magnetic field, and it relates, more particularly, to the contact pieces of the type containing disk type contact bodies wherein the contact bodies are soldered on contact carriers and have means for reducing eddy currents. In addition, the invention relates to techniques and methods for the production of the respective contact pieces.
With the increasing applications and the adaption of the vacuum switching principle to the medium voltage range, the control of larger breaking currents has become mandatory. Even for currents above 40 kA to be switched off safely, there is the concurrent requirement to maintain the outer dimensions of the switching tube unchanged or even to reduce them. A great number of specific contact geometries have been proposed, which guide the switching current in the vicinity of the contact bodies in azimuthal direction. Thereby an axial magnetic field is produced during switching between the contact pieces and thus a switching arc that is uniformly distributed and diffused over the entire contact surface is developed.
In conventional contact configurations for developing a axial magnetic field the following problems tends to occur. Due to the time variation of the current in the azimuthal current path, eddy currents are induced in closed annular contact carrier bottoms, in disk type contact bodies, or also in contact rings. Such eddy currents produce secondary magnetic fields which tend to weaken the axial magnetic field in amplitude and cause shifts in its phase relative to the current flowing through the switch. The phase shift of the axial magnetic field results in, however, that during and after the zero crossing of the current a considerable axial residual magnetic field remains. This magnetic field prevents the rapid discharge of the still existing charge carriers from the contact gap and supports an undesired re-ignition of the arc.
In the prior art various techniques and arrangements for preventing eddy currents in vacuum switches with axial magnetic field are know. In German patent document DE-PS 24 43 141, for example, a contact arrangement is disclosed in which there are four current conductors extending radially and azimuthally in the form of a hook to produce the axial magnetic field, and the disk type contact body is radially slotted to avoid eddy currents. From German patent document DE-OS 32 31 593, a different construction of a contact from azimuthal current conductors is disclosed which are formed by a multiple codirectional slotting of a pot-shaped contact, onto which is soldered a contact disk provided with radial slots, as contact body. In European patent document EP-A-00 55 008, furthermore, the course of current conductors for magnetic field production is illustrated, where the current flows in the contact disk in multiple meanders in the plane of the arc and the contact body is divided by wide slits into several parts.
In the technical literature, (for example, IEEE Transactions on Power Apparatus and Systems (1980), pages 2079 to 2085), it is stated under what conditions slits are required to prevent eddy currents in the contact disk exposed to the arc, the necessity of such slits being generally assumed in the practice.
A disadvantage in conventional arrangements because of the wide radial slits in the contact surface toward the arc cathode bases preferentially start at the edges thereof, which due to overheating may lead to re-ignition. Moreover, in the state of the art, the slotting of the disks is limited to about one third of the contact disk diameter for the purpose of maintaining the stability of the contact body. Because of the absence of slotting in the central region of the contact disks, the eddy currents flowing there remain fully effective.